The technique of using pulse compression when receiving echos from targets is earlier known; see for instance C. Elachi "SPACEBORNE RADAR REMOTE SENSING: APPLICATIONS AND TECHNIQUES", pages 57-58. The radar system transmits a pulse signal of given pulse time-length, for instance a pulse duration of 50 .mu.s. The actual signal is frequency swept, i.e. the beginning of the signal has a first given frequency f.sub.1 and the end of the signal has another signal f.sub.2. The frequency band B=f.sub.1 -f.sub.2 may, for instance, be 50 MHz. The radar is side-looking, whereby echos from various locations on the ground are reflected and arrive back at mutually different points in time.
The radar receiving lobe, i.e., the radiation pattern used when the radar is in the receiving mode, is intended to track the ground echo between two points on the earth's surface. The echo reflected from a target point on the earth's surface gives rise to a received signal in the radar, where the receiving lobe is swept over the target. The received frequency f.sub.1 will therewith arrive slightly before the frequency f.sub.2 and in the order that the frequencies f.sub.1 and f.sub.2 were transmitted from the radar system.
Pulse compression of the received frequency components of the pulse is effected in the receiver with the intention of obtaining a stronger echo and in order to obtain improved distance resolution of the target. FIG. 1 illustrates the principle of pulse compression. The received signal pulse, FIG. 1a, includes the frequencies f.sub.1, f.sub.2, f.sub.3 and is applied to a matched delay line which delays the frequencies to mutually different extents, so that the frequencies f.sub.2 and f.sub.1 will "catch up" with the frequency f.sub.3 and so that finally, FIG. 1d, all components will form a compressed pulse.
FIG. 2 illustrates the radar receiving lobe and the propagation of the transmitted pulse (broken line) at a given moment in time on the earth's surface when receiving in accordance with the known technique. When the lobe contacts a target point, the first frequency f.sub.1 is reflected and the echo is caught by the receiving lobe at point 1. The centre frequency (f.sub.1 -f.sub.2)/2 is reflected by the same target point somewhat later. Since the receiving lobe is swept and "tracks" the echo, the centre frequency will be captured by the receiving lobe at point 3. The same applies to the frequency f.sub.2, which is reflected last and which is captured by the receiving lobe at point 2. The width of the lobe is roughly equal to the propagation of the pulse.
FIG. 3 illustrates the amplitude of the received signal as a function of frequency. The amplitude is weighted, depending on the configuration of the receiving lobe. When the "outer" frequencies f.sub.1 and f.sub.2 are detected within the receiving lobe (points 1 and 2), said frequencies will be weaker than the centre frequency (f.sub.1 -f.sub.2)/2 (point 3).
Thus, the problem is that when using a narrow receiving lobe whose width is of the same order of magnitude as the "momentary angular width" of the echo, the amplitude-frequency characteristic of the echo is subjected to pronounced quadratic amplitude weighting. When the pulse is later compressed, the propagation obtained results in poorer resolution in respect of time/distance. There is also obtained a large amplification derivative at the beginning and the end of the pulse, which renders the system highly sensitive to small antenna lobe pointing errors.